1. Field of the Invention
The present invention relates to an isolated switching power supply apparatus that includes a main transformer that transmits power from a primary side to a secondary side and a power switch that interrupts a current flowing from a DC input power supply to a primary coil of the main transformer, and that outputs a desired DC voltage or DC current.
2. Description of the Related Art
FIG. 1 is a schematic circuit diagram of a DC-DC converter 58 described in U.S. Pat. No. 6,147,478. The DC-DC converter 58 is a circuit that is connected to a DC input power supply Vin and outputs a voltage to a load resistor R1. The DC-DC converter 58 includes a first power switch S1, a synchronous rectifier S2, an output smoothing capacitor Cout, resistors R1, R2 and R3, a capacitor C1, a comparator 52 having a hysteresis characteristic, and a power switch driving circuit 54.
The DC-DC converter described in U.S. Pat. No. 6,147,478 is a step-down converter that employs an improved control method of a general “hysteresis control method”. The hysteresis control method is a control method that compares an output voltage with a reference voltage by using a comparator having a hysteresis characteristic to determine the ON/OFF state of the power switch S1, and performs a switching operation by using output voltage variations due to ripples.
With a hysteresis width being denoted as HYS, when the output voltage increases due to a ripple during an ON period of S1 and exceeds reference voltage+(½)HYS, S1 is turned OFF and S2 is turned ON. When the output voltage decreases due to a ripple and falls below reference voltage−(½)HYS, S1 is turned OFF again and S2 is turned ON.
Such a DC-DC converter using a hysteresis control method has a simple control circuit configuration and an improved transient responsiveness because there is no phase delay in an error amplifier.
On the other hand, a general hysteresis control method has a disadvantage in that the general hysteresis control method is greatly dependent on ripples caused by a switching operation. As the capacitance of an output smoothing capacitor of a DC-DC converter is increased, ripples become small and the switching frequency is lowered. In addition, when ripples are too small, the control may become unstable due to the influence of noise, and as a result, the general hysteresis control method cannot be used in applications that require ripples to be of a strict standard value. Furthermore, when a low ESR capacitor such as a ceramic capacitor is used as an output smoothing capacitor, a ripple has a voltage waveform that is close to a sinusoidal wave pattern having a π/2 phase delay, rather than a ramp wave pattern, and thus the gain is reversed at the beginning of an ON period of a power switch (that is, at the time when the power switch is turned ON, the output voltage on the secondary side is changed to a falling direction), and thereby stable control cannot be performed.
Instead of using a hysteresis characteristic of a comparator, so-called “Bang-Bang control” using response delay times td1 and td2 of a comparator is also generally known. In the Bang-Bang control, during an ON period of S1, after a first response delay td1 after the output voltage has increased due to a ripple and exceeded a reference voltage, a power switch S1 is turned OFF (a synchronous rectifier S2 is turned ON), and after a second response delay td2 after the output voltage has decreased and fallen below the reference voltage, the power switch S1 is again turned OFF (the synchronous rectifier S2 is turned ON).
The Bang-Bang control has an advantage in that there is an improved transient responsiveness but the Bang-Bang control has the same problem as the hysteresis control method, which is the disadvantage of being greatly dependent on ripples in a switching operation.
In the DC-DC converter of U.S. Pat. No. 6,147,478, a ramp wave is formed by an integrating circuit composed of the resistor R1 and the capacitor C1 and an output voltage is superposed on a voltage divided between the resistors R2 and R3 to solve the foregoing problem. Thus, stable control is possible even when the ripple voltage is reduced due to the capacitance of the output smoothing capacitor having been increased or even when a low ESR capacitor is used as the output smoothing capacitor.
Despite the advantages described above, the hysteresis control system and the Bang-Bang control system have only been applied to step-down converters which are ON/ON type non-isolated DC-DC converters (forward DC-DC converters) due to the following reasons.
In ON/OFF type DC-DC converters (flyback DC-DC converters), electromagnetic energy is stored in an inductor or an exciting inductance of a main transformer during an ON period of a power switch, and the stored electromagnetic energy is released to a smoothing circuit through a rectifier during an OFF period of the power switch. Since the ripple voltage in the output smoothing capacitor decreases during the ON period and increases during the OFF period, the gain is reversed in the hysteresis control method and the Bang-Bang control method, and thereby stable control cannot be performed. That is, because there is a relationship where the output voltage on the secondary side of the main transformer decreases during an ON period of the power switch, the control direction is inverted, and thereby control becomes difficult.
Furthermore, in ON/OFF type isolated DC-DC converters in which the primary side and the secondary side are isolated from each other by a transformer, it is necessary to provide a comparator, which compares an output voltage with a reference voltage, on the secondary side of a main transformer and provide a power switch on the primary side thereof, and thus the comparator and the power switch are separated from each other with the main transformer therebetween.
For these reasons, there is a problem in that the hysteresis control method and the Band-Band control method cannot be applied to ON/OFF type isolated DC-DC converters.
Furthermore, feedback control in a voltage mode and a current mode, which are general modes in ON/OFF type isolated DC-DC converters, has many problems such as an inferior response compared to that in the hysteresis control method and the Bang-Bang control method, and a photocoupler used in transmission of error signals from the secondary side to the primary side, having pronounced transmission delay of signals, limitation of the maximum allowable temperature to approximately 100° C., and a current transfer ratio that changes over time.
In ON/OFF type isolated DC-DC converters, after all the electromagnetic energy stored in the main transformer is released to a smoothing circuit through a rectifier, a zero voltage state or a near-zero voltage state (quasi-zero voltage state) occurs due to LC resonance between the exciting inductance of the main transformer and the parasitic capacitance parallel to the power switch. When the power switch is turned ON at this timing, the switching becomes zero voltage switching (ZVC) or quasi-zero voltage switching, enabling a reduction in switching loss and noise. However, since the hysteresis control method and the Bang-Bang control method determine the turn ON timing of the power switch by comparing the output voltage with the reference voltage, zero voltage switching cannot be realized in an ON/OFF type DC-DC converter, even if the hysteresis control method or the Bang-Bang control method can be applied thereto. As a result, there is a problem that causes an increase in switching loss and noise.